In general, a great many techniques of chemical analysis are available for separating the species constituting a mixture as a function of various properties (size, mass, chemical properties, etc.). Among these technologies, mass spectrometry makes it possible to separate the species as a function of their mass. For this purpose, firstly the mixture to be analysed is subjected to ionization, which then makes it possible to separate the various compounds as a function of their mass-to-charge ratio. The ionized species are injected with a certain velocity into an electromagnetic field, which can be fixed or variable over time and in space. The forces that are exerted on the ions then modify their path as a function of their mass-to-charge ratio. This can lead to spatial or temporal separation of the different chemical species.
More particularly, the principle of a time-of-flight mass spectrometer, i.e. identifying chemical compounds as a function of measurement of a travel time specific to each species, has been known since the 1950s. The first device of this kind was described by W. Stephens in U.S. Pat. No. 2,847,576.
With reference to FIG. 1, which is a block diagram, the different ionic species m1, m2 and m3 are accelerated by an electrostatic field 10 and are then injected into a free flight zone 20 (or flight tube, which can have a length of 1 m in some devices), i.e. a field-free zone where the ions move apart (“drift zone”) as a function of their mass-to-charge ratio. The lightest masses (m1) arrive at the detector 30 placed at the exit from this free flight zone 20 before the heaviest masses (m2, m3).
Measurement of the time of flight thus gives the value of the mass, as shown on the right of the figure.
Preliminary ionization, which takes place in a source 3, can be effected by desorption-ionization by means of a laser 5, but other ionization techniques exist.
Various techniques have been developed for greatly increasing the performance of this type of spectrometer. They involve for example modifying the architecture of the device, and adding elements for correcting the dispersions.
Thus, an electrostatic mirror 40, called a reflectron, is present in some mass spectrometers at one point or another in the flight zone. Said mirror 40, shown at the bottom of FIG. 1, employs a static electric field for changing the direction of the paths of the charged particles. It is conventionally composed of a series of discrete electrodes 42 isolated from one other and on which electrostatic potentials decreasing from one electrode to another are applied, thus creating a potential gradient along the path of the ions.
This mirror 40 is necessary for obtaining good resolution, in particular for devices of small dimensions in which the temporal and spatial dispersions cannot be ignored.
On the one hand, it can compensate for the spatial dispersion of the ions connected with the geometric extent of the ion source 3. Ions of identical mass can in fact be generated at different points of the source 3, which causes a random temporal dispersion, reducing the resolution of the system.
On the other hand, the mirror 40 enables ions having the same mass-to-charge ratio, but different kinetic energies, to arrive at the detector 30 at the same time. There is a position, downstream of the reflectron 40, where the time penalty imposed on the most energetic ions exactly compensates the advantage that the latter had initially over the less energetic ions. It is at this temporal focusing plane of the ions with identical m/z ratio that the detector 30 is positioned (not shown in this bottom part of FIG. 1).
More and more areas, in particular environmental monitoring, civil security and the chemical and petrochemical industries, have an increasing need to be able to perform measurements for analysis of the environment in situ, in real time and with stringent requirements in terms of performance. Mass spectrometers would be a useful measuring means for meeting this need for analysis, but existing devices are either bulky and expensive laboratory items, or portable instruments, rarely autonomous, with degraded performance.
Various problems are encountered on the route to miniaturization of mass spectrometers. On the one hand, it is necessary to develop techniques for manufacture of the special elements of the spectrometer that are reliable and are not too expensive. On the other hand, it is necessary to have a technique for acquisition of the signals collected at the exit of the device that is compatible with the miniature character of the distances, the travel times and optionally of the quantities of material injected into the instrument. Finally, it is necessary to develop solutions to problems encountered specifically at the micrometric scale, in particular related to distortion of the electromagnetic fields.
Regarding the methods of manufacture, teams are working on the miniaturization of mass spectrometers using MEMS technologies (MEMS: MicroElectroMechanical Systems). These techniques have led to the emergence of a new type of miniature components, such as sensors, actuators, or sources of energy.
Patents and scientific articles have been published in recent years on ion traps, quadrupolar filters or miniature magnetic filters, some of which have also been manufactured using MEMS technology. Significant patents in this field have the numbers U.S. Pat. No. 7,402,799, U.S. Pat. No. 6,967,326, U.S. Pat. No. 6,469,298 (corresponding to EP-1 218 921), U.S. Pat. No. 7,208,729 and U.S. Pat. No. 7,217,920. The article “Complex MEMS: a fully integrated TOF micro mass spectrometer” by Eric Wapelhorst et al., which appeared in Sensors and Actuators, A 138 (2007) pp 22-27, for its part describes a mass spectrometer in MEMS technology comprising a monochromatic temporal filter that only searches for a single ionic species. The complete spectrum of the chemical species present is obtained by applying a voltage ramp to the temporal filter. Multiple injections are therefore required, which takes a considerable time. The document “Fabrication of a novel micro time-of-flight mass spectrometer” by H J Yoon et al., which appeared in Sensors and Actuators, A 97-98, (2002), pp 41-447, finally describes a rudimentary time-of-flight mass spectrometer in MEMS technology, the very low resolution of which means that it can only distinguish ions of very small masses. In particular, it is noteworthy that this spectrometer does not comprise a reflectron, development of which at a small scale presents considerable difficulty.
The articles “A miniature MEMS and NEMS enabled Time-of-Flight Mass Spectrometer for Investigations in Planetary Science” by Roman et al., which appeared in Proceedings of SPIE, Vol. 6959, 1st Jan. 2008, pages 69590G1-G13 and “Simulation of a Miniature, Low-Power Time-of-Flight Mass Spectrometer for In Situ Analysis of Planetary Atmospheres” by King et al. (more or less the same authors as for the preceding article, but in a different order), which appeared in Proceedings of SPIE, Vol. 6959, 1st Jan. 2008, pages 69590E1 to E15, describe spectrometers which, although comprising elements of small size of the MEMS type, involve complex assembly of various components resulting in an assembly whose dimensions are not in the micrometric range, since the prototype that is described has dimensions of 5 cm in height, 10 cm in width, and 30 cm in length; this considerable length is due in particular to the time-of-flight chamber.
Moreover, regarding the electronic systems for signal acquisition, the advances made in recent years have resulted in the marketing of portable instruments permitting sampling of signals at several tens of billions of samples per second for a pass-band of several tens of GHz.
In the context of a mass spectrometer of the time-of-flight type, for which miniaturization results in envisaging differences in time-of-flight between particles of similar masses of less than a nanosecond, these new generations of sampling systems make reliable measurement of the signals possible, if effective separation has been carried out.
Finally, although there have been advances in the techniques for the manufacture of the specific spectrometer components and in the digital acquisition devices, the systems that have been offered still have a small analysis range as well as a limited resolution m/Δm (m being the mass of the ion for which the resolution is expressed, Δm the minimum mass difference measurable in the region of mass m), of the order of 10 to 50 for ions having a mass of only about a hundred atomic mass units.
Thus, the analysis is limited to chemical compounds of low mass and the resolution and sensitivity are low. Moreover, the devices are not very robust.
U.S. Pat. No. 7,605,377 describes a reflectron with discrete electrodes which is miniature, and can be used in a time-of-flight mass spectrometer. It comprises a substrate, on the surface of which the electrodes are present, individually secured by connecters formed in the substrate or coupled to the latter.
Nonetheless, in this device, the decrease in ratio between the dimensions of the discrete electrodes, including their distance apart, and the diameter of the ion beam, means that the distortion of the electric field in the lateral zones of the reflectron affects the ion beam.
In fact, as is shown schematically at bottom right in FIG. 1, the electric field is not uniform in the space between two electrodes. This defect of uniformity is maintained when the linear dimensions of the reflectron are changed (i.e. assuming the potential differences between the electrodes are altered in such a way as to maintain a constant potential gradient).
As the diameter of the ion beam is not changed, the result is that the defect of uniformity of the field, which was without consequence in a macroscopic device, becomes a major drawback in a microscopic device. The electric field component in the Z direction attracts the ions in the Z direction, therefore causing the ion beam to diverge. The ions that have diverged too much from their path strike the electrodes of the reflectron and are consequently lost for the analysis.
This problem greatly complicates the design of a time-of-flight mass spectrometer of small size, which is to have satisfactory resolution and sensitivity, incorporating a reflectron of this type with discrete electrodes.
A reflectron lens is known for conventional spectrometers of large size used in the laboratory and is described in U.S. Pat. No. 7,154,086 and US-2010/0090098. This device consists of a glass tube or a tube made of some other material on which a layer of glass is deposited, held by a single end at the bottom of a flight tube of the spectrometer. Such a device using a lens that is held at a single end is not compatible with the use of a substrate, on the surface of which the electrodes are present, individually secured by connecters formed in the substrate or coupled thereto, for example by stacking.
Such a device using a lens is also incompatible with a support having a function of mechanical support for manipulation of the device by a user in the volume of which the reflectron is integrated, for example by stacking. Moreover, the manufacturing techniques, the geometries and the materials used are not compatible with the methods of micro-fabrication of the microelectronics industry. It is therefore necessary to develop another fabrication technology for this element, which is indispensable for the development of a micro-mass spectrometer.